The present invention relates generally to terrain following aircraft control, and more particularly to a method for accessing and utilizing terrain elevation data in the context of terrain following flight.
To minimize above ground elevation, and therefore minimize visibility and vulnerability to ground based detection and attack, military aircraft often execute terrain following flight. Terrain following flight generally maintains a given elevation above ground level independent of actual elevation above sea level. In other words, the aircraft follows the ground contour at a substantially fixed elevation above the ground and maneuvers according to prevailing ground contour along a given flight path. The actual above ground level elevation may increase to establish a suitable climb angle to clear an upcoming high elevation terrain feature.
The general algorithm applied to flight following terrain is to select the tallest terrain feature lying along and near the flight path. The aircraft altitude and attitude vector are referenced to determine whether or not an upcoming terrain feature will be cleared. If necessary, the terrain following algorithm requires that the aircraft enter a suitable climb angle to clear any upcoming terrain features. Otherwise, the algorithm would maintain a substantially consistent above ground elevation according to a given terrain profile data structure.
As used herein, the term "terrain profile" shall refer to a data structure representing terrain along a given flight path. A terrain following algorithm uses the terrain profile data structure to execute terrain following flight along the designated flight path. A terrain profile may be thought of generally as a terrain cross-sectional, elevational contour, e.g., as taken through a vertical plane containing the flight path, but must account for terrain conditions near the designated flight path. For example, for a given point along the flight path, the highest elevation point laterally outward on either side a given distance is assigned as the terrain profile elevation at the given point. This produces a conservative, i.e., safe, elevation contour in the terrain profile.
Most terrain following applications use radar and/or ladar sensor data to generate a terrain profile along a predicted flight path of the aircraft. The predicted flight path is generally based on the current aircraft attitude and velocity. Terrain following flight is preferably executed, however, with limited or no active sensor data because active sensor emissions make the aircraft visible to threat installations at greater distances. Using active sensors, especially at high power, compromises covert missions because the aircraft can be detected at long distances by hostile forces. If active sensors are to be used, such sensors are preferably used at low power settings to minimize detectable emissions and allow only short range detection by hostile forces. Unfortunately, terrain following flight requires more distant terrain information, typically exposing the aircraft to detection if generated using long range high power active sensors. Furthermore, active sensor data has its limitations. Sensor data alone cannot see behind hills or around corners. Sensors can only "guess" where to gather terrain data in generating a terrain profile.
Digital terrain elevation data represents surface elevation at discrete "data posts." Each data post has a surface location or address, e.g. latitude and longitude, and an associated elevation, e.g. relative to sea level. Thus, a simple form of a DTED database would deliver a scalar elevation datum in response to longitude and latitude address input. More complicated DTED databases have been developed for certain applications. For example U.S. Pat. No. 4,899,293 issued Feb. 6, 1990 to J. F. Dawson and E. W. Ronish shows a tessellation method for creating a spherical database by warping a digital map, including digital terrain elevation data, by longitude and latitude parameters.
DTED database systems are used in flight mission computer systems and flight planning strategy in military applications aid in, for example, covert and evasive flight operations. As used in mission computer systems, a DTED database can aid a pilot in time-critical maneuvers such as terrain following flight or in selecting routes evasive with respect to a given threat position. Such threat positions may be known in advance, or detected while in flight. The computation speed required in accessing and calculating routes or alternatives based on DTED can be vitally critical, especially for repeated computations required to keep a pilot fully appraised of current terrain conditions and route alternatives. Thus, improvements in methods of accessing DTED and computations based on extracted DTED are not simply improvements in computational elegance, but can be life-saving and critical to mission success.
Terrain profiles have been built by extracting a massive volume of DTED data with reference to a designated flight path. As may be appreciated, each data sample taken from the DTED database for consideration in generating the terrain profile requires a given amount of processing time. The data extracted from the DTED database for generating the terrain profile corresponded to data posts lying along a length of the flight path preceding aircraft position and all data posts within a given distance of that length portion, i.e. a fixed length and width region of data posts along the flight path and identified relative to current aircraft position. The terrain profile must provide safe, conservative information. To this end, a large volume of DTED data has been incorporated into terrain profiles. Unfortunately, the volume of data extracted and processed has constrained terrain profile generation, i.e., has required excess terrain profile calculation time.
Thus, prior methods of generating terrain profiles include long range active sensors, but long range sensor emissions make aircraft visible at long distances, and DTED database systems, but generating conservative terrain profiles requires massive DTED data points and can require relatively long calculation time.
It is desirable, therefore, that terrain following flight be executed without aid of high power, long range active sensor data to avoid exposing the aircraft to threat installations. It is further desirable that a method of producing a terrain profile for executing terrain following flight support dynamic and efficient calculation time.